Zoe, Lassie and Harvey head into the high country with Graham when the waters of Diamond Lake begin to mysteriously recede, threatening the animals who drink from the lake. Graham fears that some obstruction may have blocked the flow of water. When the rangers discover a logjam that has dammed the creek that feeds the lake, they set to work busting it up... never knowing that Graham has become trapped in an underground channel that will flood the moment the logjam is broken.